1. Field of the Invention
The present invention relates to hearing aids and methods of processing sound signals in hearing aids. The invention further relates to controlling sound signals and, more particularly, to methods and hearing aid devices that process sound signals, in particular for hearing impaired persons by controlling input levels of band split compressors in a hearing aid.
2. The Prior Art
Hearing loss of a hearing impaired person is quite often frequency-dependent. This means that the hearing loss of the person varies depending on the frequency. Therefore, when compensating for hearing losses, it can be advantageous to utilise frequency-dependent amplification and compression in a wide dynamic range. Hearing aids therefore often provide to split an input sound signal, and especially speech signals received by an input transducer of the hearing aid, into various frequency intervals, which are also called frequency bands. In this way it is possible to adjust the input sound signal of each frequency band individually depending on the hearing loss in that frequency band. The frequency dependent adjustment is normally done by implementing a band split filter and a compressor for each of the frequency bands, so-called band split compressors, which may be summarized to a multi-band compressor. In this way it is possible to adjust the gain individually in each frequency band depending on the hearing loss as well as the input level of the input sound signal in a respective frequency band. For example, a band split compressor may provide a higher gain for a soft sound than for a loud sound in its frequency band.
In order to adjust the hearing loss of a person by frequency, it is advantageous to split the signal into a large number of frequency bands. However, when using frequency-dependent amplification and compression, care must be taken to avoid unnecessary distortions often associated with multi-band non-linear processing. A particular problem of frequency-dependent amplification and compression is the so-called spectral smearing which may cause a loss of speech intelligibility since, e.g., the spectral differences in the speech spectrum are smeared or smoothed out due to the individual gain adjustments of the various band split compressors. A way to cope with this problem would be to reduce the number of frequency bands, however, this carries a disadvantage since it will then not be possible to provide a detailed frequency-dependent compensation of a hearing loss of a hearing impaired person.
U.S. Pat. No. 6,873,709 describes hearing aid devices that provide improved filtering and compression of sound signals. The described method and apparatus attempt to achieve a better speech audibility and intelligibility at low levels and also to pre-serve spectrum contrast at high levels by constraining the gain amount for each of the frequency bands against gain amounts associated with at least one neighbouring frequency band based on the corresponding estimated signal levels. As a result, the input sound signals will not be amplified by the gain amount adjusted by the compressors but with a constrained gain amount. This means that at first each band split compressor controls the actual initial gain in the respective frequency band based on the estimated signal level in this frequency band. After the gain adjustment by each individual compressor the initial gain amounts are constrained by a succeeding gain constraint unit if the initial gain amount exceeds a certain threshold level. Nevertheless, there remain disadvantages with speech audibility and intelligibility since the subsequent constraining of the individual initial gain amounts cannot really cope with the spectral smearing associated with the multi-band non-linear processing in the individual band split compressors. The restricted capability of constraining the initial gain amounts becomes even more apparent by the fact that a gain amount is constrained only if the signal level in the frequency band exceeds the threshold level since by this a spectrum contrast only with respect to higher signal levels will be preserved. The implementation of a gain constrained unit therefore may not cope with spectral smearing in all cases.
Thus, there is a need for improved techniques for providing multi-band compression processing of sound signals.